Cathode bottom for producing aluminum

ABSTRACT

A cathode bottom, to a method for the production thereof, and to the use thereof in an electrolytic cell for producing aluminium. The cathode bottom has at least two cathode blocks and/or at least one cathode block and at least one sidewall block, which are arranged at a distance from each other. The gap is filled with a pre-compressed graphite plate, composed of expanded graphite and a graphite intercalation compound.

The present invention relates to a cathode bottom, to a method for the production thereof and to the use thereof in an electrolysis cell for producing aluminium.

Aluminium is generally produced by means of fused-salt electrolysis in electrolysis cells. An electrolysis cell generally comprises a trough made of sheet iron or steel, the bottom of which is lined with heat insulation. In said trough, up to 24 cathode blocks made of carbon or graphite that are connected to the negative terminal of a power source form the floor of another trough, the wall of which consists of side-wall bricks made of carbon, graphite or silicon carbide. A gap is formed between two cathode blocks in each case. The arrangement of the cathode block and the gap, which may be filled, is generally referred to as the cathode bottom. The gaps between the cathode blocks are conventionally filled with ramming mass consisting of carbon and/or graphite based on coal tar. This serves as a seal against molten constituents and to compensate for mechanical stresses during start up. Carbon blocks that hang from a support frame connected to the positive terminal of the power source are generally used as the anode.

In an electrolysis cell of this kind, a molten mixture of aluminium oxide (Al₂O₃) and cryolite (Na₃AlF₆), preferably approximately 2 to 5% aluminium oxide, approximately 85 to 80% cryolite, and other additives, is subjected to fused-salt electrolysis at a temperature of approximately 960° C. In the process, the dissolved aluminium oxide reacts with the solid carbon anode and forms liquid aluminium and gaseous carbon dioxide. The molten mixture covers the side walls of the electrolysis cell with a protective crust, while the aluminium accumulates on the floor of the electrolysis cell underneath the molten material on account of the greater density of the aluminium compared with the density of the molten material, so as to be protected against reoxidation caused by oxygen in the air. The aluminium produced in this manner is removed from the electrolysis cell and further processed.

During electrolysis, the anode is used up, whereas the cathode bottom behaves in a largely chemically inert manner throughout. The anode is therefore a wearing part that is exchanged during the operating time, whereas the cathode bottom is designed for long-term and prolonged use. Nevertheless, current cathode bottoms are subject to wear. Mechanical abrasion of the cathode surface occurs on account of the aluminium layer moving over the cathode bottom. Furthermore, (electro)chemical corrosion of the cathode bottom occurs on account of aluminium carbide formation and sodium intercalation. Since, in general, 100 to 300 electrolysis cells are connected in series so as to form an economical installation for producing aluminium, and since an installation of this kind is generally intended to be used for at least 4 to 10 years, the failure and replacement of a cathode block in an electrolysis cell in an installation of this kind can be expensive and require costly repair work, which significantly reduces the economic viability of the installation.

A disadvantage of the above-described electrolysis cell comprising the ramming mass consisting of carbon and/or graphite based on coal tar is that thin layers of the coarse-grained ramming mass cannot be produced for technical reasons, such as mechanical stability or the ramming procedure, and therefore gaps are present which reduce the cathode surface area and in which aluminium and particles that increase the wear of the cathode bottom can intercalate.

The most widely used anthracite ramming masses are less electrically and thermally conductive than graphitised cathode blocks in particular. This reduces the effective cathode surface area and higher energy consumption results from the greater overall resistance, which decreases the economic viability of the process. Furthermore, the wear of the cathode bottom increases on account of the higher specific load.

Another problem is that ramming masses often contain binders based on coal tar that contain polycyclic aromatic hydrocarbons. These are toxic and/or carcinogenic. During use, some of these or of the pyrolysis products enter the atmosphere.

In WO 2010/142580A1, the ramming mass is replaced with a compressible graphite film, as a result of which substances in the ramming mass that are harmful to health, such as polycyclic aromatic hydrocarbons, can be dispensed with, and sealing between the cathode blocks of the cathode bottom can be achieved.

However, the deformation behaviour changes with respect to that which is ideal on account of, for example, reusing the steel trough of an electrolysis cell, such that additional cracks, fissures or dislocations of entire cathode blocks occur, as a result of which the sealing cannot be guaranteed. Since predicting the deformation behaviour is often difficult, said additional cracks, fissures or dislocations are an operational risk, since aluminium or electrolyte melt can in this case leak out, which can even lead to immediate failure of the cell. For this reason, the additional cracks and/or fissures must be compensated for.

The object of the present invention is thus to provide a cathode bottom which can compensate for the deformation behaviour of the electrolysis cell and thus ensure sealing. In the context of the present invention, a cathode bottom is understood to mean not only the arrangement of at least two cathode blocks leaving an optionally filled gap, but also the arrangement of at least one cathode block and at least one side-wall brick leaving an optionally filled gap. A gap is the space between two cathode blocks or a cathode block and a side-wall brick.

This object is achieved by a cathode bottom for an electrolysis cell for producing aluminium, comprising at least two cathode blocks and/or at least one cathode block and at least one side-wall brick, which are arranged at a predetermined distance from one another, the gap being filled with a filler that can be pre-arranged on at least one cathode block or side-wall brick, characterised in that the filler is a pre-compressed graphite plate consisting of expanded graphite and a graphite intercalation compound.

According to the invention, the cathode bottom comprises a filler that is arranged on at least one cathode block and/or a side-wall brick and that is characterised in that the filler comprises a pre-compressed plate based on expanded graphite and a graphite intercalation compound. Within the meaning of the present invention, “pre-compressed” means that the plate based on expanded graphite and a graphite intercalation compound has been compressed but can be compressed further. This means that the pre-compressed plate based on expanded graphite and a graphite intercalation compound is partially compressed and is thus pressed and can also be pressed further.

According to this invention, the pre-compressed graphite plate based on expanded graphite and a graphite intercalation compound is also referred to as a pre-compressed graphite plate. These two terms are interchangeable within the meaning of the present invention and refer to a pre-compressed graphite plate made of expanded graphite and a graphite intercalation compound.

Expanded graphite has the following advantageous properties: it is harmless to health, environmentally compatible, soft, compressible, lightweight, resistant to ageing, chemically and thermally resistant, technically gas- and liquid-tight, non-combustible and easily workable. Furthermore, expanded graphite forms no alloy with liquid aluminium. It is therefore suitable as a filler for a cathode bottom for an electrolysis cell for producing aluminium.

In order to produce graphite having a vermiform structure, graphite, such as natural graphite, is usually mixed with an intercalate such as an inorganic acid, for example nitric acid, sulfuric acid or mixtures thereof, and thus a graphite intercalation compound is obtained as the intermediate product, which is then heat-treated at an elevated temperature of, for example, 600° C. to 1200° C. (DE10003927A1). The intercalation of the acid typically occurs in the presence of an oxidising agent, for example nitric acid (HNO₃), hydrogen peroxide (H₂O₂), potassium permanganate (KMnO₄) or potassium chlorate (KClO₃).

Expanded graphite is a graphite which is expanded with respect to natural graphite by a factor of 80 or more, for example, in the plane perpendicular to the hexagonal carbon layers. Expanded graphite is characterised by excellent formability and good interlockability on account of the expansion. Expanded graphite may be made into sheet form, with thermal conductivities of up to 500 W/(m-K) being achieved.

The thermal conductivity is determined using the Ängstrom method (“Ängström's Method of Measuring Thermal Conductivity”; Amy L. Lytle; Physics Department, The College of Wooster, Theses).

The intercalate of a graphite intercalation compound may be an electron donor or electron acceptor, preferably an electron acceptor. “Electron donor” is understood according to this invention to be compounds or elements that have free electrons, for example lithium, potassium, rubidium or caesium. “Electron acceptor” is understood according to this invention to be a compound which comprises an electron gap, i.e. an incomplete noble gas configuration.

Metal halides, preferably metal chlorides, of the elements iron (Fe), aluminium (Al), antimony (Sb), tin (Zn), yttrium (Y), chromium (Cr) or nickel (Ni) and acids, preferably sulfuric acid (H₂SO₄), acetic acid (CH₃COOH) and nitric acid (HNO₃), or mixtures of sulfuric acid/nitric acid and sulfuric acid/acetic acid, may be selected as electron acceptors in the context of the invention. Preferably, aluminium halides, particularly preferably aluminium chlorides, or sulfuric acid (H₂SO₄) are used as electron acceptors.

The use of the pre-compressed graphite plate as the filler makes it possible to close cracks or fissures that arise during the process or reuse of the steel trough by expanding the graphite intercalation compound, which expansion depends on the prevailing temperatures. In this way, a kind of “self-healing” of the fissures or cracks is possible.

Possible defects or fissures caused by installation may also be healed by the expansion of the salt, and gaps between possible abutting edges, which when using pre-compressed graphite plates, which are smaller than the full cathode length, are minimised.

As a result, fissures or cracks, inter alia, can also be closed in inaccessible regions of the cathode. By closing the additional fissures and/or cracks, sealing of the electrolysis cell is achieved.

According to the invention, various graphite intercalation compounds can also be mixed together that show the beginning of the expansion on account of the different intercalates at different temperatures relative to one another. In this way, various temperature regions of the cell, for example between the cathode blocks and between the cathode and side-wall brick, can be covered in a targeted manner.

As a result, it is possible to provide a customised filler.

Advantageously, the proportion of expanded graphite in the pre-compressed graphite plate is between 70 and 99.5 wt. %, preferably between 80 and 95 wt. % and particularly preferably 90 wt. %, and the proportion of graphite intercalation compound in the pre-compressed graphite plate is between 0.5 and 30 wt. %, preferably between 5 and 20 wt. % and particularly preferably 10 wt. %. The constituents, i.e. the expanded graphite and the graphite intercalation compound, together always make up 100 wt. %.

If the proportion of graphite intercalation compound in the pre-compressed graphite plate is less than 0.5 wt. %, too few cracks are closed, since too little of the graphite intercalation compound, which can subsequently expand, is present, and thus the graphite intercalation compound may be in the wrong place on account of the limited distribution near the surface.

If the graphite intercalation compound proportion in the pre-compressed graphite plate is over 30 wt. %, the stability of the pre-compressed graphite plate is too low, since the pre-compressed graphite plate achieves stability on account of the already expanded graphite particles interlocking.

If the proportion of graphite intercalation compound in the pre-compressed graphite plate is 0.5 to 30 wt. %, the aforementioned self-healing of the fissures and/or cracks is possible; i.e. remaining fissures or cracks are closed by means of the subsequent expansion of the graphite intercalation compound at the prevailing temperatures of the electrolysis cell. A filler that is adapted to the temperature program of the electrolysis cell and that is thus customised can be provided by means of the selection of the graphite intercalation compound.

Another advantageous effect is the physiological harmlessness of the pre-compressed graphite plate compared with the conventional coal-tar-containing carbon composition, which contains polycyclic aromatic hydrocarbons that are harmful to health. Furthermore, the pre-compressed graphite plate has a higher electrical and thermal conductivity compared with the conventional coal-tar-containing carbon composition and thus also increases the effective cathode surface area.

The pre-compressed graphite plate used according to the invention can be inserted into the regions of an electrolysis cell in which conventional ramming mass is used, i.e. in particular in gaps which are formed between cathode blocks, but also in spaces between side walls of the electrolysis cell and cathode blocks. The pre-compressed graphite plate is used in particular as a sealing means between cathode blocks of a cathode bottom and between the cathode block and side wall of a cathode bottom.

The filler and the cathode blocks or cathode block and side wall are connected in a frictional manner and preferably end flush. The filler and cathode block or side wall may optionally be adhesively bonded, for example by means of a phenolic resin. In this invention, the terms side wall and side-wall brick are used analogously.

By using a pre-compressed graphite plate instead of conventionally used coal-tar-containing ramming mass, the width of the gap between cathode blocks can be reduced and thus the effective cathode surface area can be increased. The material is used as a filler between the two cathode blocks that is not only capable of sealing the gap between the two cathode blocks but is also capable, on account of the compressible nature thereof, of compensating for swelling of the cathode blocks and/or side-wall bricks caused by sodium expansion, which occurs during electrolysis. The sodium enters the cathode blocks and/or side-wall bricks by diffusing out of the molten cryolite (Na₃AlF₆).

According to the invention, the pre-compressed graphite plate therefore has a thickness of 2 to 35 mm, preferably 5 to 20 mm, particularly preferably 10 to 15 mm. A minimum thickness of 2 mm is required in order to be able to compensate for the sodium expansion of the cathode block and/or side wall.

According to the invention, the pre-compressed graphite plate has a density of 0.04 to 0.5 g/cm³, preferably 0.05 to 0.3 g/cm³, particularly preferably 0.07 to 0.1 g/cm³. The density must be less than 0.5 g/cm³, such that a graphite plate having a thickness of 2 mm is produced at a typical weight per unit area of 1000 g/m³. Said graphite plate can be further compressed such that there is no gap formation between the cathode block and/or side wall.

In another preferred embodiment, the filler is arranged on two opposing surfaces of a cathode block that adjoin the surface that forms the gap, and on and in the gap, such that the filler is flush. The fact that the filler is flush means that, within the meaning of the invention, the filler is arranged on the cathode blocks such that the cathode bottom in each case has uniform dimensions along the length, height and width thereof. In a cathode bottom in an electrolysis cell, there is a space between the side walls of the electrolysis cell and the cathode blocks. The filler is in this case arranged such that it fills the gaps between the cathode blocks and the regions between the cathode blocks and the side walls. The cathode bottom thus forms the entire floor of the electrolysis cell, i.e. it extends up to all side walls of the electrolysis cell, the cathode bottom having regions of higher thermal and electrical conductivity in the form of cathode blocks and regions of lower thermal and electrical conductivity in the form of the filler material consisting of expanded graphite and a graphite intercalation compound.

The cathode blocks preferably have a larger length than width dimension, whereas the width and height dimensions are approximately equal. In general, cathode blocks are up to 3800 mm long, 700 mm wide and 500 mm tall. Preferably, the at least two cathode blocks are arranged such that the length dimensions thereof are parallel. The predetermined distance between two cathode blocks is usually approximately 30 to 60 mm. A reduction of the distance between cathode blocks is possible by using the filler according to the present invention. Therefore, when using 650 mm wide cathode blocks, for example, the distance between cathode blocks must be at least 40 mm when using conventional ramming masses as the filler between said cathode blocks, whereas said distance can be reduced to 10 mm when using the pre-compressed graphite plate.

Therefore, the effective cathode-block surface area is increased by approx. 5% when 40 mm wide gaps between 650 mm wide cathode blocks are reduced to 10 mm, for example.

Preferably, the at least one cathode block comprises at least one means for connection to a power source. For example, the cathode block comprises at least one recess for receiving a conductor rail, which can be connected to a power source. If at least two cathode blocks are oriented such that the length dimensions thereof are parallel, the recess is preferably oriented in the longitudinal direction of the cathode block, i.e. the recess extends in parallel with the gap formed between two cathode blocks. Of course, the cathode bottom may further comprise a composite element between the cathode block and the conductor rail, for example a contact mass or the like.

The at least one cathode block is designed to be electrically and thermally conductive, resistant to high temperatures, chemically stable with respect to bath components of the electrolysis and unable to form an alloy with aluminium. The cathode block is preferably made of graphite and/or amorphous carbon. Particularly preferably, the cathode block comprises graphite or graphitised carbon, because these more than other materials meet the requirements in respect of thermal and electrical conductivity and chemical resistance for forming a cathode bottom in an electrolysis cell for producing aluminium.

The cathode bottom, in the preceding preferred embodiments having the at least two cathode blocks and/or at least one cathode block and at least one side-wall brick, comprises regions that have a high conductivity, and in those having the filler comprising the pre-compressed graphite plate, comprises regions which generally have a lower conductivity than the cathode blocks and/or side-wall bricks, but that are capable of sealing the gaps formed between the cathode blocks such that no bath components can penetrate into lower regions of the cathode bottom during electrolysis.

The two components, i.e. cathode blocks and side-wall bricks, and the pre-compressed graphite plate, therefore fulfil various functions of the cathode bottom. On account of its multifunctional design, said cathode bottom can therefore be sized for large-scale use. On account of the arrangement of a plurality of cathode blocks and/or cathode blocks and side-wall bricks, a large conductive cathode surface is produced and on account of the effective sealing of the gaps between the cathode blocks using the pre-compressed graphite plate, wear of and damage to the cathode surfaces between the cathode blocks are prevented.

The cathode bottoms according to the invention may be produced according to a method comprising the following steps:

a) providing at least one cathode block;

b) arranging a filler on at least one surface of the at least one cathode block, the filler comprising at least one pre-compressed plate based on expanded graphite and a graphite intercalation compound;

c) arranging at least one other cathode block or at least one side-wall brick at a predetermined distance from the at least one cathode block such that the filler fills a gap that is formed by arranging the other cathode block or side-wall brick at the predetermined distance from the at least one cathode block.

By producing a cathode bottom that comprises a pre-compressed graphite plate, a high effective cathode surface area can be achieved by it being possible to arrange a plurality of cathode blocks one next to the other.

The cathode block is produced such that the filler is connected to the at least one cathode block in an interlocking manner by means of said filler being arranged on said cathode block; if necessary, an adhesive is additionally used.

By arranging the other cathode block or side-wall brick on the cathode block, firstly, another interlocking connection between the cathode blocks or between the cathode block and the side-wall brick is achieved by means of the pre-compressed graphite plate. The arrangement of the other cathode block or side-wall brick is achieved by means of hydraulic or mechanical pressing, optionally using adhesive, and thus a frictional connection is produced. By means of the method according to the invention, it is possible to reduce the width of the gap between the cathode blocks or between the cathode block and the side-wall brick compared with conventional gap widths and thus to increase the effective cathode surface area. The pre-compressed graphite plate that fills the gap is partially reversibly compressible, such that it can compensate for the swelling of the cathode blocks.

After arranging the other cathode block, a pre-compressed graphite plate is received in the gap, the graphite plate being a slightly resilient filler that seals the gap without forming cavities. The step of arranging at least one other cathode block may be carried out before or after arranging the filler on the at least one cathode block.

The cathode blocks may be provided with means that allow connection thereof to a power source before or after they are installed. For example, a cathode block may be provided with at least one recess before or after installation, in which recess at least one conductor rail is inserted which can be connected to a power source. Furthermore, a cathode block handled in this manner may be provided with other means before or after installation, for example a contact mass may be arranged between the cathode block and the conductor rail.

The cathode bottom according to the invention is used in an electrolysis cell for producing aluminium. In a preferred embodiment, the electrolysis cell comprises a trough which generally comprises sheet iron or steel and has a round or quadrangular, preferably rectangular, shape. The side walls of the trough may be lined with carbon, carbide or silicon carbide. Preferably, at least the floor of the trough is lined with heat insulation. The cathode bottom is arranged on the floor of the trough or on the heat insulation. At least two, preferably 10 to 24, cathode blocks are arranged in parallel with one another in relation to the length dimension thereof at a predetermined distance from one another such that a gap is formed between each block that is filled, in each case, with at least one pre-compressed graphite plate. The spaces between the side walls and the cathode blocks are filled either with a filler that comprises a pre-compressed graphite plate, or with a conventional anthracite ramming mass. Likewise, the gaps between the cathode blocks may be filled either with a pre-compressed graphite plate or with a conventional anthracite ramming mass. Each gap of the cathode bottom may be filled differently. The cathode blocks are connected to the negative terminal of a power source. At least one anode, for example a Soderberg electrode or a prefired electrode, hangs from a support frame that is connected to the positive terminal of the power source, and projects into the trough without touching the cathode bottom or the side walls of the trough. Preferably, the distance from the anode to the walls is greater than to the cathode bottom or the forming aluminium layer.

In order to produce the aluminium, a solution of aluminium oxide in molten cryolite is subjected to fused-salt electrolysis at a temperature of approximately 960° C., the side walls of the trough being covered in a solid crust of the molten mixture, while the aluminium accumulates underneath the molten material because the aluminium is denser than said molten material.

Other features and advantages of the invention are described below with reference to the following drawings, without being limited thereto,

in which:

FIG. 1 is a schematic cross-sectional view of a cathode bottom according to the invention;

FIG. 2 is a schematic cross-sectional view of part of an electrolysis cell for producing aluminium that comprises a cathode bottom according to the invention;

FIGS. 3a to 3c schematically show a method sequence for producing a cathode bottom according to the invention; and

FIGS. 4a to 4c schematically show another method sequence for producing a cathode bottom according to the invention.

FIG. 1 is a schematic cross-sectional view of a cathode bottom 1 according to the invention. The cathode bottom 1 comprises a filler 3 consisting of a pre-compressed graphite plate that fills a gap 5 formed between two cathode blocks 7. The cathode blocks 7 have an electrical and thermal conductivity that is sufficient for use in fused-salt electrolysis and are manufactured from graphitised carbon, for example. The cathode blocks 7 each comprise a recess 9 for receiving a conductor rail (not shown) that makes possible connection of said cathode blocks to a power source. The filler 3 and the cathode blocks 7 end flush.

FIG. 2 is a schematic cross-sectional view of part of an electrolysis cell 213 for producing aluminium. The electrolysis cell 213 comprises a trough 215 made of steel. The side walls 217 of the trough 215, one of which is shown in FIG. 2, are lined with side-wall bricks 219 made of graphite, one of which is shown in FIG. 2. The floor of the trough 215 is lined with a heat-insulating layer 221 so as to be completely covered thereby. A cathode bottom 21 is arranged on the heat-insulating layer 221. The cathode bottom 21 comprises a filler 23 and cathode blocks 27, two of which are shown in FIG. 2, which are arranged at a predetermined distance from one another. In standard electrolysis cells, the filler 24 arranged between the side-wall brick 219 and the cathode block 27 is a ramming mass consisting of carbon. The gap between the side-wall brick 219 and the cathode block 27 is filled in this way. According to the invention, the filler 24 may also be a pre-compressed graphite plate. The filler 23 also comprises a pre-compressed graphite plate. A gap 25 is formed between each cathode block 27. The filler 23 fills the gap 25, and the ramming mass 24 fills the relevant space between the cathode block 27 and the side wall 217 such that the heat-insulating layer 221 is completely covered by the cathode bottom 21 comprising the ramming mass 24, the filler 23 and the cathode blocks 27. As shown in FIG. 2, the filler 23 ends flush with the cathode blocks 27. The cathode blocks 27 each comprise a recess 29 suitable for receiving a conductor rail (not shown) that can be connected to a negative terminal of a power source (not shown). Furthermore, the electrolysis cell 213 comprises anodes 223, two of which are shown in FIG. 2, which each hang from a support 225 connected to a positive terminal of a power source (not shown). A solution 227 of aluminium oxide in molten cryolite is located in the electrolysis cell 213. During electrolysis, aluminium 229 accumulates between the solution 227 and the cathode bottom 21.

FIGS. 3a to 3c schematically show a method sequence for producing a cathode bottom 31 according to the invention.

FIG. 3a shows the provision of two cathode blocks 37 each having a recess 39 for receiving conductor rails that are arranged at a predetermined distance from one another such that a gap 35 is formed. FIG. 3b shows the filler 33 comprising a pre-compressed graphite plate being inserted into the gap 35. FIG. 3c shows the cathode bottom 31 as it can be used in an electrolysis cell for producing aluminium. The filler 33 fills the gap 35. The amount dimensions of the filler 33 are selected such that the filler 33 ends flush with the cathode blocks 37 and completely fills the gap 35. It should be noted that possible connections and connecting means of the cathode bottom 31 to a power source have been omitted from FIGS. 3a to 3c for the sake of clarity.

FIGS. 4a to 4c schematically show another method sequence for producing a cathode bottom 41 according to the invention.

FIG. 4a shows the provision of a cathode block 47 that comprises a recess 49 for receiving a conductor rail (not shown). FIG. 4b shows the filler 43 comprising a pre-compressed graphite plate being arranged on a surface of the cathode block 47 in a planar manner, an adhesive optionally being used to secure said filler. FIG. 4c shows another cathode block 47 comprising a recess 49 being arranged on the filler 43 such that said the other cathode block is frictionally connected to the cathode block 47 by means of the filler 43. FIG. 4c shows the cathode bottom 41 as it can be used in an electrolysis cell for producing aluminium. By repeating the steps shown in FIGS. 4b and 4c , a cathode bottom comprising a plurality of cathode blocks arranged one next to the other can be produced. It should be noted that possible connections and connecting means of the cathode bottom 41 to a power source have been omitted from FIGS. 4a to 4c for the sake of clarity.

In the following, the present invention is described on the basis of embodiments, whereby the embodiments do not limit the invention.

EMBODIMENT 1

50 g sulfuric acid (95-98%) and 1 g H₂O₂ (70%) are added to 20 g graphite. After an intercalation time of 20 minutes has elapsed, the reaction mixture is suction-filtered, washed with distilled water (approx. 250 ml) in several portions and suction-filtered once more. The graphite intercalation compound obtained was dried at 120° C. to constant weight. Subsequently, 90 wt. % of the graphite intercalation compound obtained is expanded at approximately 1000° C. 10 wt. % of the graphite intercalation compound is added to the expanded graphite obtained in this manner by means of continuous distribution of the graphite intercalation compound onto a layer of expanded graphite particles, which are then immediately compressed.

EMBODIMENT 2

50 g sulfuric acid (95-98%) and 1 g H₂O₂ (70%) are added to 20 g graphite. After an intercalation time of 20 minutes has elapsed, the reaction mixture is suction-filtered, washed with distilled water (approx. 250 ml) in several portions and suction-filtered once more. The graphite intercalation compound obtained was dried at 120° C. to constant weight. Subsequently, 90 wt. % of the graphite intercalation compound obtained is expanded at approximately 1000° C. and directed through a chute onto a conveyor belt. In said conveying chute, 10 wt. % of the graphite intercalation compound is supplied continuously in a ratio of 1:9. Subsequently, compression immediately takes place.

LIST OF REFERENCE NUMERALS

-   1 cathode bottom -   3 filler -   5 gap -   7 cathode block -   9 recess -   21 cathode bottom -   23 filler -   24 ramming mass -   25 gap -   27 cathode block -   29 recess -   31 cathode bottom -   33 filler -   35 gap -   37 cathode block -   39 recess -   41 cathode bottom -   43 filler -   47 cathode block -   49 recess -   213 electrolysis cell -   215 trough -   217 side wall -   219 side-wall brick -   221 heat-insulating layers -   223 anode -   225 support -   227 solution of aluminium oxide -   229 aluminium 

1-10. (canceled)
 11. A cathode bottom for an electrolysis cell for producing aluminium, comprising at least two cathode blocks and/or at least one cathode block and at least one side-wall brick, which are arranged at a predetermined distance from one another, the gap being filled with a filler that can be pre-arranged on at least one cathode block or at least one side-wall brick, characterised in that the filler is a pre-compressed graphite plate consisting of expanded graphite and a graphite intercalation compound.
 12. The cathode bottom according to claim 11, characterised in that the proportion of expanded graphite in the pre-compressed graphite plate is between 70 and 99.5 wt. %.
 13. The cathode bottom according to claim 11, characterised in that the proportion of graphite intercalation compound in the pre-compressed graphite plate is between 0.5 and 30 wt. %.
 14. The cathode bottom according to claim 13, characterised in that the intercalate of the graphite intercalation compound is an electron acceptor in the form of an acid selected from the group consisting of sulfuric acid (H₂SO₄), acetic acid (CH₃COOH) or nitric acid (HNO₃), or mixtures of sulfuric acid/nitric acid and sulfuric acid/acetic acid.
 15. The cathode bottom according to claim 11, characterised in that the pre-compressed graphite plate has a thickness of 2 to 35 mm.
 16. The cathode bottom according to claim 11, characterised in that the pre-compressed graphite plate has a density of 0.04 to 0.5 g/cm³.
 17. The cathode bottom according to claim 11, characterised in that the filler is arranged on two opposing surfaces of a cathode block and/or side-wall brick that adjoin the surface of the cathode block that forms the gap, and on and in the gap, such that the filler is flush.
 18. A method for producing a cathode bottom according to claim 11, comprising the following method steps: a) providing at least one cathode block; b) arranging a filler on at least one surface of the at least one cathode block, wherein the filler comprises at least one pre-compressed plate based on expanded graphite and a graphite intercalation compound; c) arranging at least one other cathode block or at least one side-wall brick at a predetermined distance from the at least one cathode block such that the filler fills a gap that is formed by arranging the other cathode block or side-wall brick at the predetermined distance from the at least one cathode block.
 19. The method according to claim 18, characterised in that the arrangement of the filler on the at least one surface of the at least one cathode block comprises securing said filler to the surface by means of an adhesive.
 20. A use of a cathode bottom according to claim 11 in an electrolysis cell for producing aluminium. 